1. Field of the Invention
This invention relates to an aluminum substrate for making magnetic recording media, said substrate having formed thereon an anodic oxide film or layer which is hard, has high heat resistance, and has substantially no black spot defect.
2. Description of the Prior Art
Recently, ther has been much demand to increase the recording density of magnetic recording media, such as magnetic disks, etc.
For responding to such demands, it is necessary to reduce the thickness of the magnetic medium layer to be formed on a surface of the substrate and the spacing between the magnetic head and the magnetic medium.
The substrate of such magnetic recording media is required to have more excellent surface properties with respect to smoothness, and hardness.
An aluminum alloy having formed on the surface an anodic oxide film layer is used as a substrate for magnetic recording media such as magnetic disks for high recording density.
The reason why such an aluminum substrate having an anodic oxide film is used for high recording density magnetic recording media is that the anodic oxide film is hard and has high wear resistance, shows good polishability to easily provide smooth surface in high accuracy, and hence a thin magnetic layer can be easily formed on the surface thereof.
Hitherto, such an aluminum substrate used for the purpose is generally prepared by applying an anodic oxidation treatment by a sulfuric acid process to the surface of an aluminum alloy containing about 2 to 6% magnesium to easily form a hard film having good polishability but a conventional aluminum substrate finished by such a sulfuric acid anodization process has the important defects, which prevent any increase in the recording density of the magnetic recording media.
One of these defects is microscopic pit-form defects on the anodic film, which is called "black spot". The existence of this defect on the anodic film of the aluminum substrate hinders the formation of the magnetic layer on the substrate, which results in the occurrence of signal error at the use of the magnetic recording medium. Accordingly, it is desired to reduce the number of such defects on the film to as few as possible and also reduce the size of the defects to below 3 .mu.m.
Another defect is the occurrence of cracks in the anodic oxide film formed on an aluminum substrate. In the case of obtaining a high-density magnetic recording medium, a very thin layer of .alpha.-Fe.sub.2 O.sub.3 (under 0.5 .mu.m) is formed on the aluminum substrate by sputtering or other appropriate method and then the substrate is heated to a temperature of 300.degree. to 400.degree. C. to convert .alpha.-Fe.sub.2 O.sub.3 into .gamma.-Fe.sub.2 O.sub.3. In the case of performing such a high-temperature heating, the anodic oxide film on the substrate is cracked which reduces the quality of the product. For avoiding the occurrence of these cracks, it is necessary to reduce the thickness of the anodic oxide film below 3 .mu.m. However, if the aluminum substrate having such an thin anodic film is used for magnetic disks, there occurs a problem of reducing the headcrash resistance of the magnetic disk owing to the reduction of its strength for pressure resistance.